This paper assesses how the ‘Great Replacement’ conspiracy theory (the idea that ethnically homogeneous populations in European nations are being ‘replaced’ by people of non-European origin) is articulated online by three different actors. By analyzing argument patterns and multimodal features of the cases, the paper shows that the conspiracy theory is a flexible political discourse that can be used strategically by both far-right and mainstream right-wing actors. It highlights the role of affect in online communication, and particularly how anti-immigration actors feed off circulating emotions such as insecurity and fear among the citizenry. The results show that processes of demographic change, caused by immigration, are negatively politicized through the use of pseudo-scientific sources, historic narratives of ethnic homogeneity, threat frames, visual fear appeals and other elements that constitute the wider conspiracy theory of an ongoing ‘replacement’ of native populations. The paper argues that the mainstreaming of conspiracy claims and theories related to immigration poses a threat, not only to democratic institutions and societies, but also to people of immigrant backgrounds.